The growth cone at the distal tip of the extending axon is a specialized sensory apparatus that transduces extracellular signals into growth along appropriate pathways to correct synaptic targets. Its proper function is crucial to nervous system development and hence adult nervous system performance. This application seeks a molecular understanding of the signal transduction mechanisms at the neuronal growth cone. Particular emphasis is focused on the action of the Semaphorin family of proteins recently recognized to inhibit axonal extension and terminal arborization. Previous work from this laboratory and others has led to the understanding that class 3 Semaphorins bind to cell surface Neuropilins and that a Semaphorin/Neuropilin complex activates a Plexin transmembrane polypeptide to initiate an intracellular signal transduction cascade. This growth cone repulsive transduction cascade involves the GTP-binding protein racl and CRMPs. Other classes of Semaphorins activate Plexins directly, without Neuropilin involvement. Here, we seek to extend this understanding in several directions. Given that the 20-member Semaphorin family affects numerous biological events and that the Plexin family has at least 9 members, specificity of action is a crucial issue. Ligand/receptor pairing and biological functions will be explored for the class 3 and 4 Semaphorins and various Plexins. The molecular mechanisms whereby the function of downstream elements, racl and CRMP, is altered by the intracellular domain of Plexin will be investigated through a combination of protein binding studies, enzymatic assays and cell morphology assays. We will examine the modulation of this signal transduction both by ligand/receptor clustering and by receptor protein tyrosine phosphatase (RPTP) control of Plexin phosphorylation. Together these experiments should provide a detailed description of the molecular events that underlie growth cone responsiveness to extracellular repulsive signals such as the Semaphorins. Knowledge of these pathways will lay the necessary groundwork for understanding the pathophysiology of human developmental abnormalities of the brain. The same mechanisms are likely to function during adult nervous system regeneration and plasticity, so that pharmacological modulation of these systems may potentiate recovery after traumatic injury and improve function in degenerative neurologic diseases.